1. Field of the Invention
This invention is related to wellbore milling processes, wellbore milling tools, and cutting inserts for such tools.
2. Description of Related Art
Milling tools are used to cut out windows or pockets from a tubular, e.g. for directional drilling and sidetracking; and to mill out for removal materials downhole in a wellbore, such as pipe, casing, casing liners, tubing, or jammed tools (a "fish"). The prior art discloses various types of milling or cutting tools provided for milling out a fish or for cutting or milling existing pipe or casing previously installed in a well. These tools have cutting blades or surfaces and are lowered into the well or casing and then rotated in a milling/cutting operation. With certain tools, a suitable drilling fluid is pumped down a central bore of a tool for discharge beneath the cutting blades or surfaces and an upward flow of the discharged fluid in the annulus outside the tool removes from the well cuttings or chips resulting from the cutting operation.
Milling tools have been used for removing a section of existing casing from a well bore to permit a sidetracking operation in directional drilling, to provide a perforated production zone at a desired level, to provide cement bonding between a small diameter casing and the adjacent formation, or to remove a loose joint of surface pipe. Also, milling tools are used for milling or reaming collapsed casing, for removing burrs or other imperfections from windows in the casing system, for placing whipstocks in directional drilling, or for aiding in correcting dented or mashed-in areas of casing or the like.
The prior art discloses a variety of cutting inserts for wellbore milling tools. Certain of these inserts have a surface irregularity, recess, or indentation that serves as a chipbreaker to break a cutting being produced by an insert to limit the length of the cuttings. Certain prior art inserts have multiple chipbreakers on a single insert. Many prior art inserts with multiple chipbreakers have chipbreaking indentations spaced closely together so that a crack or break at the location of one chipbreaker easily propagates through the insert body affecting other adjacent chipbreakers and resulting in the cracking, breaking or destruction of the adjacent chipbreakers.
There has long been a need for an efficient and effective milling method in which damaging effects on one chipbreaker indentation are isolated from other chipbreaker indentations. There has long been a need for tools with such inserts. There has long been a need for milling methods using such tools and such inserts.